Progress in FY2019 was in the following areas: (1) APPLICATION OF NEW RAPID MIXING/FREEZING TECHNOLOGY TO PEPTIDE FOLDING AND SELF-ASSEMBLY. In 2018, we perfected an apparatus for rapid mixing and freezing that enables studies of transient intermediates in structural conversion processes on the millisecond time scale. In 2019, we completed and published an initial application of this technology (Jeon et al., PNAS 2019), to the folding and tetramerization process of the bee venom peptide melittin following a rapid pH jump (from pH 3 to pH 7). 2D solid state 13C-13C NMR spectra of selectively isotopically labeled melittin samples were recorded with evolution times (time between initiation of the pH jump and initiation of freeze-quenching) between 2.2 ms and 30 ms. These spectra show the evolution from an unfolded, random-coil-like state to a structurally ordered state. Helix formation by melittin molecules occurs in about 9 ms. Intermolecular crosspeak signals in the 2D spectra indicate formation of helix-helix contacts on the same time scale, so that folding and self-assembly apparently occur concurrently. However, full structural order, indicated by progressive sharpening of crosspeak signals from amino acid sidechains, develops more slowly, on a time scale of approximately 60 ms. We ascribe this slower process to gradual annealing of sidechain-sidechain packing within melittin tetramers. These experiments demonstrate how solid state NMR measurements (with sensitivity enhancements from dynamic nuclear polarization) can provide new information about molecular mechanisms of biomolecular structural conversion processes. Applications to initial stages of amyloid peptide oligomer formation, to binding-induced folding in peptide/protein complexes, and to globular protein folding are planned. (2) MICRON-SCALE MRI: The goal of this project is to obtain magnetic resonance images with isotropic resolution of about 1 micron by using low temperatures and dynamic nuclear polarization to enhance MRI sensitivity. In 2018, we demonstrated 2.8 micron resolution at low temperatures, but without DNP. In 2019, we published a paper that describes how nitroxide-based DNP polarizing agents (i.e., dopants) affect NMR spin relaxation times at temperatures in the 5-30 K range (Chen and Tycko, J. Phys. Chem. B, 2018). This paper shows that temperatures below 10 K are required to suppress local magnetic field fluctuations due to electron spin flip-flop transitions, which otherwise lead to short NMR relaxation times under DNP conditions. Taking this information into account, we have adapted our MRI apparatus for operation at 5 K with minimal liquid helium consumption. We have also developed non-uniform sampling and image processing methods that allow total image acquisition times to be reduced by a factor of five, thus alleviating problems associated with long DNP build-up times at very low temperatures. To further reduce image acquisition times, we have developed a novel method for slice selection that is based on radio-frequency phase modulation under Lee-Goldburg homonuclear decoupling conditions. With these methods in place, we plan to acquire the first 1.5 micron resolution images at the beginning of FY2020. If successful, this work will represent an eight-fold reduction in voxel volumes relative to the best previous results.